This invention relates to an improved control circuit for a high voltage DC power supply and more particularly to an improved control circuit which detects incipient arcing between a high voltage electrode, such as a charging electrode in an electrostatic coating system, and a grounded object and shuts off the high voltage power supply and grounds the electrode prior to arcing.
Electrostatic coating systems generally comprise a high voltage DC power source which is connected between a grounded object which is to be coated and a spray gun or similar device which directs an electrostatically charged atomized liquid coating or an electrostatically charged cloud of particulate coating material towards the object. There is a danger of fire or explosion when operating electrostatic coating systems. This danger exists due to the flammability of solvents within liquid coatings and the flammability of clouds of particulate coatings along with the risk of ignition by a spark between the coating apparatus and electrical ground. Typically, objects to be coated are conveyed past a fixed or reciprocating coating applicator. If the objects move too close to the coating applicator or should swing on the conveyor, a spark may occur between the coating applicator and the grounded object possibly resulting in a fire or an explosion.
Various approaches have been used in the past to prevent arcing from occurring in electrostatic coating systems. One external method for preventing fires or explosions involves the use of trip or sensing wires or photo detection devices. When a grounded object moves too close to the high voltage, the external sensing device is triggered and causes the high voltage at the power supply to be shut off while the charge is removed from the spray device by connecting it to ground. Another external system involves the use of an infrared sensing device to detect flames after a fire has occurred. Gases such as Freon then are released in the ignition area in order to quench the flames.
Another approach for reducing the risk of fire or explosion is to use an internal control circuit. One such control circuit comprises a very high value resistance connected in series between the high voltage power source and the coating material charging electrode. In the event of an incipient arc, the resistor, which may be on the order of five or six hundred megohms, limits available energy to a level insufficient for producing ignition. However, when an object approaches the spray gun or coating implement during normal spraying operations and there is a normal current increase, there will be a voltage drop across the high value resistor which results in a lower voltage at the spray tip or material charging electrode. This type of approach has lower charging levels of the spray or coating material and, consequently, a lower transfer efficiency of the coating material to the target or object being coated than a non-resistive power supply.
Another internal safety method is embodied in a feedback system. The current in the ground leg of the high voltage power supply is utilized to monitor the output current in the high voltage output line to the spray gun. One such use of this method is seen in an overcurrent shut down circuit. Such systems have been in use for over 15 years in the electrostatic coating field. The current within the ground leg of the high voltage DC circuit passes through a resistor which produces a low voltage signal proportional to the current. When the low voltage signal reaches a preset reference level which is used to indicate an incipient arc condition, the input voltage to the high voltage producing circuit is interrupted and, simultaneously, the high voltage output and spray device are grounded through the use of relays, solenoids or pneumatic switches. A circuit of this type works quite well where ther is a gradual current increase in the high voltage circuit until the preset trip current is reached. However, the circuit will not respond sufficiently fast to a rapid change in conditions, such as may occur when an object swinging on a conveyor is moved past an electrostatic spray gun. Also, a control of this type does not operate well where a number of spray guns are operated from a single high voltage power source. The control circuit must be set sufficiently high so as not to shutdown the power supply during normal maximum current loads from each of the spray guns. When an incipient arc exists at only one of the spray guns and the currents to the remaining spray guns is less than their maximum permissable currents, the circuit may not shut down the power supply even though incipient arcing exists.
An improved feedback control system for a high voltage power supply is illustrated, for example, in U.S. Pat. Nos. 3,851,618, 3,875,892, 3,894,272, 4,075,677 and 4,187,527. The improved system periodically samples the discrete DC current level in the high voltage circuit and stores each sample for a specific time interval. Each stored sample is compared to the next periodic sample. The difference between the two samples over the predetermined sampling timed interval represents a rate of change of the current in the high voltage circuit, or the di/dt. If the rate of change of the DC current exceeds a preset rate, the control circuit disconnects or shuts down the high voltage power supply and simultaneously grounds the high voltage output. This circuit is operated in conjunction with the above described overcurrent limiting circuit which shuts down the power supply when some predetermined maximum current occurs within the high voltage circuit. A control circuit which measures the rate of current change or the di/dt is a significant improvement over prior art high voltage control circuits for preventing arcing or sparking. However, the circuit does not always respond sufficiently fast to a sudden and very rapid current change or to a relatively small current change such as occurs when a small object moves rapidly towards an electrostatic spray gun. Another situation where response may be inadequate is when a small grounded projection extends from a large object which is being conveyed past an electrostatic coater. The current level within the high voltage system is determined primarily by the large mass of the object and the small grounded projection may move within arcing distance from the coating apparatus before the circuit can respond. This is particularly true in systems which look at the rate of change in current over a period of time rather than constantly looking at the instantaneous current within the system. Another problem with the di/dt method is when a grounded object moves very slowly to within proximity of the electrostatic coating apparatus and may even come to rest at the edge of the arcing distance. The di/dt at this time is not of practical use since the sensitivity setting to recognize such a condition would cause continuous false shutdown to the production system. Even more basic is the fact that the above condition has barely any change in current with time until arcing actually occurs.